This invention relates to thermosetting imidazole cured epoxy-polysulfide rubber compositions particularly suited for use as automotive body solders. More particularly, the invention relates to epoxy-polysulfide rubber based compositions containing a filler system of aluminum powder, an ion treated clay, a wetting agent for the clay, a fibrous or plate-like talc, and optionally, fumed silica. The system provides a composition with excellent physical properties.
Sheet metal automotive body panels are generally joined by welding their overlapped depression edges. The resulting trough-like joints are then filled in with lead based solder.
Considerable effort has been directed toward replacing lead solder with a lighter, less expensive polymeric material. However, such replacement must be as adaptable as lead to assembly line use. That is, the polymeric solder should lend itself to rapid application and cure. It cannot sag on a vertically oriented surface at either room or elevated cure temperatures. The cured solder must be readily sandable and form a smooth feather edge with adjacent sheet metal. It must be compatible with automotive finishing systems and able to withstand paint oven temperatures up to about 200.degree. C. without sagging or degrading. The painted solder must be resistant to delamination or flaws brought about by corrosion, impact and thermal cycling.
We know of no resinous body solder, other than those described herein, which meets these requirements. Certain combinations of epoxy resin, flexibilizers, and solid imidazole curing agents have shown some promise as substitutes for lead solder. See, for example, U.S. Pat. No. 3,923,571, relating to dispersed solid imidazole curing agents in liquid or semisolid epoxy compositions. However, these body solder systems are deficient in at least two critical respects. First, if the uncured resins are thickened enough to prevent sag on vertical surfaces, they are too tacky to be applied with a trowel or other tool. Instead they require the use of a removable plastic backing sheet for application to a car body; and second, unless substantial amounts of toxic lead or chromate containing anti-corrosion pigments are added to the epoxies, the solders suffer drastic decreases in strength with time, especially under high temperature or humidity conditions. The patent suggests that the solders may be applied at thicknesses of only 4 millimeters or less and that fillers, plasticizers, and other resin additives may be incorporated in the basic composition without any special consideration.
We have discovered that the type, amount, and even the shape of filler constituents are important to achieving workable body solders with all of the above mentioned desirable characteristics. More specifically, we have discovered a unique system of filler constituents which in combination with each other provide imidazole cured epoxy-polysulfide rubber resins with unexpected sag resistance, spreadability, adhesion to sheet metal, resistance to failure under impact, corrosion resistance and more.